Latent image printing

ABSTRACT

The disclosed ink composition is particularly suitable for lithographic (wet) offset printing and comprises a quick set vehicle mixed with an iron-complexing agent. This composition provides a storable latent (i.e. invisible or concealed) image when printed on a properly selected low iron-content paper. Treatment of the printed latent image with an iron salt develops the image and makes it clearly visible. Visible material can be printed with the latent material on the same paper using a conventional 2-color offset press.

United States Patent [191 Buerkley et a1.

[ Nov. 26, 1974 i 1 LATENT IMAGE PRINTING [75] Inventors: Donald D.Buerkley, Inver Grove Heights Village; Heinz E. Lange, St. Paul, both ofMinn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[22] Filed: Aug. 5, 1971 [21] Appl. No.: 169,524

[52] U.S. Cl 106/21, 35/9 G, 106/22, 106/27, 106/32, 117/367, 117/368[51] Int. Cl C09d 11/00 [58] Field of Search 106/145, 21-22,

[56] References Cited UNITED STATES PATENTS 3,108,896 10/1963 Owen106/21 X 3,147,134 10/1961 Clark 117/36.9 3,149,993 1/1963 Baumann et a1117/369 3,169,065 2/1965 Sorkin et a1. 101/457 3,262,806 7/1966 Gourge106/31 X 3,363,337 l/l968 Skinner et al 35/36 3,451,143 6/1969 Thomas eta1. 35/9 G 3,666,502 5/1972 Erickson 106/22 Primary Examiner-Joseph L.Schofer Assistant ExaminerT. S. Gron Attorney, Agent, or Firm-Alexander,Sell, Steldt & DeLaHunt [5 7] ABSTRACT 12 Claims, No Drawings LATENTIMAGE PRINTING FIELD OF THE INVENTION This invention relates to theprinting of latent, i.e. concealed or substantially invisible, imagescomprising a color-forming reactant which can be rendered visible with acoreactant metal salt or the like. An aspect of this invention relatesto educational aids or tools such as work sheets, self-answeringexamination sheets, and other paper-based feedback systems. A furtheraspect of this invention is that these educational aids can containvisible printed material also, generally as statements or questions towhich the latent material provides feedback or answers.

DESCRIPTION OF THE PRIOR ART Most of the development in the art oflatent image printing has centered around educational uses. Examples ofrecently developed paper-based systems for teaching skills through thedevelopment or revelation of latent or concealed images are: US. Pat.Nos. 3,363,337 (Skinner et al.), issued Jan. 16, 1968; 3,451,143 (Thomaset al.), issued June 24, 1969; and 3,516,177 (Skinner), issued June 23,1970. The most promising prior art systems appear to be those whichutilize an invisible ink (i.e. ink containing a substantially colorlessdye precursor or color-forming reactant) which can be printed on apaper-like sheet and developed at any suitable time with a color-formingcoreactant. Typical invisible inks are disclosed in the aforementionedSkinner et al. Pat. No. 3,363,337, and in U.S. Pat. No. 3,438,927(Ehrlich), issued Apr. 15, 1969.

Despite the rapid development of the latent printing art, many problemsremain to be solved. A problem which is often encountered with prior artinvisible inks is a phenomenon called clueing. Clueing occurs when alatent image is not, or ceases to be, totally beyond detection by thenaked eye. Typical causes of clueing include: (1) any distinctionbetween the back ground areas and the latent print areas, e.g. in lightreflectance or color (some invisible ink compositions are subject toyellowing with age, resulting in a contrast with a white paperbackground), and (2) inadvertent development of the latent image byimpurities in the ink, the paper, or the printing equipment.

Even if this'clueing problem is minimized, slow and inefficientdevelopment of the latent image may still be a problem, particularlywhen the more convenient developing means are used, e. g. imagingcrayons compris- To date, the art has apparently not developed aninvisible (latent pn'nt) ink suitable for conventional lithographicoffset printing methods, the available invisible inks being generallyhigh volatility, low viscosity, low solids-content, at least partiallyhydrophilic compositions suitable for printing methods other thanlithographic offset. Possible reasons why a lithographic latent printink has not hitherto been available are: the inability to use existingoffset presses without extensive modification; the inability to find avehicle and/or a color-forming reactant adaptable to conventionallithographic offset techniques; the inability to avoid or design aroundundesirable chemical side reactions caused by conventional metal saltdriers, particularly in view of the tendency of phenolic color-formingreactants to inhibit oxidation and/or polymerization of conventionaloffset vehicles; the inability to use 2-color offset presses, thusnecessitating a plurality of printing operations for visible andinvisible textual material; and the inability to prevent invisible inkpenetration and migration without resorting to resin-coated, lowporosity, or other high reflectance sheets which can cause clueing (seethe previously cited Ehrlich patent).

Accordingly, this invention contemplates a solution to these problemsthrough, inter alia, the formulation of a latent print ink suitable forconventional lithographic offset techniques and the selection of asuitable paper for the printing surface.

Briefly, this invention involves (1) milling a suitable 0 solidpolyhydroxyaromatic compound having the abiling an iron salt in a waxmatrix. Many educational aids require more or less instant feedback ofthe answer. Long reaction times for converting the latent, substantiallycolorless image to a colored, visible reaction product hinder theattainment of this requirement.

A further problem is the need for convenient printing methods. Spiritduplication (see the Thomas et al. patent, previously cited) is suitableonly for short runs up into the tens or hundreds of copies. Letterpressor ity to complex iron salts to form a strongly colored complex (e.g. anester of gallic acid and a C C alcohol), the milling being carried outuntil this compound is in the usual ink pigment size range (i.e..untilthe numerical average particle size lies in the range of 0.5 5 microns),preferably when the milling elements contain iron or steel, in thepresence of an iron inhibitor such as citric acid, and (2) dispersingthe milled ironcomplexing, color-forming, polyhydroxyaromatic compoundin a color stable (i.e. non-yellowing) lithographic quick set vehicle.The resulting viscous lithographic ink can be extended or modified inviscosity through the use of an aliphatic hydrocarbon oil, preferably apetroleum distillate or the like which boils at a temperature of atleast 200C. and preferably higher than 225C. The ink can be furthermodified with flatting agents and rub resistance agents such asparticulate (e.g. fumed or precipitated) silica. It is also preferableto add minor amounts of wax and polar organic additives to the ink toadjust tack and consistency and provide faster inking of a lithographicprinting plate.

The ink of this invention thus comprises, in essence, a particulatecolor-forming reactant, preferrably a light colored, solid, particulate,water-insoluble colorforming reactant capable of forming a stronglycolored complex with a coreactant metal salt, said reactant dispersedthroughout a quick set vehicle, suitably modified, if desired, by any ofthe previously described additives. Due to the small particle size ofthe color-forming reactant material, the particles remain in suspensionor dispersion and are prevented from settling, much as a pigment for alithographic ink is prevented from settling. At least 40%, preferably atleast by weight of the ink composition is the quick set vehicle. Toprovide latent images which can be subsequently developed at sufficientspeed, at least 10% and preferably 20 35% by weight of the inkcomposition should be the dispersed color-forming reactant particles.The lithographic ink of this invention permits the use of highlydesirable conventional lithographic printing methods to print bothvisible and invisible entries on a suitable offset grade paper. Standard2-color lithographic press equipment can be used to avoid the necessityof printing the visible and latent (i.e. invisible) images in separateoperations. The result obtained by this invention is a printed form onoffset grade paper which has printed visible entries and printed latentor invisible entries which can be developed with a suitable organic orinorganic iron salt-dispensing means, e.g. a chemical pen or wax crayon.These printed forms have the previously described utility in the fieldof education. A feature of these printed forms, mostimportant from thestandpoint of educational uses, is that they are not likely to beaffected by the phenomenon of clueing, described previously, providedthe paper is properly selected. It is a further feature of thisinvention that the iron content of the offset grade paper be less than0.05%, preferably less than 0.03%, based on the weight of the sheet. Asfurther aids to the elimination of clueing, the sheet preferably has aSheffield smoothness number greater than 160 (preferably greater than230) and can contain randomly distributed fibers capable of fluorescingunder ultraviolet light. Other features of the invention will becomereadily apparent from the following detailed description. Surprisingly,the advantageous effects of this invention can be obtained without anyof the operating difficulties which might be expended from the use oflithographic offset printing methods to print a latent image. Forexample, it has proved unnecessary to modify existing offset presses inany substantial manner to achieve the objects of this invention.Furthermore, the use of metal salt driers (e.g. the cobalt or manganesesalts normally added to lithographic inks) has been eliminated in thepresent invention by using a quick set vehicle. This result issurprising in view of the fact that the polyhydroxyaromaticcolor-forming reactant, like other phenolic compounds, would be expectedto inhibit oxidation and/or polymerization of the quick set vehicle,resulting in excessive penetration of the color-forming reactant intothe paper sheet and loss of the ability to rapidly produce a legibleimage upon treatment of the paper surface with a suitable iron salt. Theability to eliminate the metal salt driers is highly advantageous, sincethese cobalt and manganese compounds have the ability to tie up thecolor-forming reactant and drastically reduce its effectiveness for thecontemplated educational uses.

DETAILED DESCRIPTION OF THE INVENTION The term quick-set offset ink" orquick-set lithographic vehicle is a commonly used term in inktechnology. As pointed out in Ink Technology for Printers and Studentsby E. A. Apps (published 1963), page 187, quick-set lithographic inksare based on a twophase vehicle consisting of a stiff component and athin component. The vehicle separates into these two phases on the papersurface; the thin component is absorbed into the stock, and the stiffcomponent is left to set on the paper surface; the ink films set firm ina short period of time. Apps also points out that these inks requireless driers than do conventional inks, and drying is faster and moreuniform. A more detailed description of lithographic vehicles includingtypical quick-set vehicles is contained in the article Resins forPrinting Inks by J. W. Dyer, American Ink Maker, pages 5768 and 116(May, 1970). This article describes a wide variety of inks and inkvehicles, including the most common quick-set types; i.e., the rosinester, maleic resin, phenolaldehyde, isomerized rubber, polyamide, andpolyurethane types. Any of the common types of quick-set vehicles areuseful in this invention, provided that they are reasonably color-stableand do not tend to yellow with age. For example, some of thephenol-aldehyde type quick-set vehicles are subject to yellowing over aperiod of time.

As is apparent from the description in the Apps reference, the quick-setoffset inks, regardless of their chemical composition, work on the sameprinciple. The thin component (e.g. a low volatility hydrocarbon oiland/or a flowable low molecular weight natural or synthetic polymer) isreadily absorbed by the paper, while the stiff component (e.g. a highermolecular weight natural or synthetic resinous material) is relativelynonflowable and is not absorbed. The result is a phase sep aration, thethin component being filtered away in the capillaries of the paper andthe stiff component being solidified on the surface of the paper andbinding together the ink pigment particles.

In conventional lithographic inks of the quick-set type, a drier isrequired despite this phase-separation effect. It has been theexperience of the printing industry that the quick-setting or phaseseparation effect is insufficient by itself to produce a fully driedprinted entry on the paper in a sufficiently short period of time.Furthermore, a quick-set vehicle can have the tendency to carry somepigment into the paper stock. This pigment migration effect is highlydetrimental in the art of latent or invisible ink printing, as ispointed out by the 3,438,927 (Ehrlich) patent, previously cited.Ehrlichs approach to the migration problem is to print on a resin-coatedsheet. Although the resin coating can prevent migration, as taught byEhrlich, it can also provide a relatively smooth or glossy backgroundfor the invisible entry and create conditions which tend to favorclueing. The present invention uses an approach diametrically opposed toEhrlichs in that offset grade paper, a highly absorbent type of paper,receives the printed invisible entry. For reasons which are not entirelyclear, the undesirable migration effects described by Ehrlich are not aproblem in this invention.

Further properties of quick-set vehicles and inks, as is well known inthe art, are tack, high viscosity (generally in the range of 300 3,500poises at room temperature), hydrophobicity, and pigment-carryingcapacity. By hydrophobicity" is meant that the ink composition, andpreferably each of the primary components included in it, have little orno solubility in water and are resistant to emulsification by water. Asa general rule, lithographic inks are incapable of being emulsified inwater to an extent greater than about 20% by weight, although a verysmall amount of emulsification (e.g. 10 20%) is permissible and evendesirable. However, the quick-set vehicle itself is oleophilic oroleoresinous in nature and is therefore hydrophobic, as is any aliphatichydrocarbon oil used as an extender or viscosity adjusting agent. In thepresent invention, the relatively small amount of iron inhibitor (e.g.citric acid) and aliphatic carboxylic acid (e.g. oleic acid, used to getfaster inking up of the printing plate) and silica filler, do notsignificantly alter the hydrophobic characteristics of the ink.

It has been found that the polyhydroxyaromatic colorformingv reactantused in this invention should also be somewhat hydrophobic, and for thisreason a hydrophobic substituent such as an aliphatic hydrocarbonradical having more than four carbon atoms should be substituted eitherdirectly on the aromatic nucleus or through an ester or carbonyllinkage. The ester and carbonyl linkages are strongly preferred fortheir relatively low level'of dermitological activity as compared to theaforementioned directly substituted aliphatic radicals.

. .carbon atoms, preferably 6-l 8 carbon atoms. When the -COOR radicalis present, as in the gallic acid esters, it has also been found thatthe use of a R with at least six carbon atoms appears to contribute tothe color stability of these esters. The reason for the increased colorstability cannot be readily explained, and although this invention isnot bound by any theory, it is theorized that the esters of gallic acidwith higher aliphatic alco- 1 hols are more resistant to oxidation orother chemical reactions which can cause undesirable color changes. Afurther benefit of introducing the higher molecular weight R radicalsinto the polyhydroxyaromatic molecule is that the room temperature vaporpressure of the compound is reduced, resulting in greater shelf life forthe invisible entries printed on the examination sheets, workbooks, orthe like. There appears to be no theoretical upper limit on the numberof carbon atoms in R except that the imaging speed can be slowed down asthe molecular weight increases. Furthermore, the availability ofalkanols or other suitable aliphatic raw materials containing more than18 carbon atoms is very limited.

Accordingly, preferred polyhydroxyaromatic compounds of this inventionare those having two or three ortho hydroxyls such as suitablederivatives of gallic acid, pyrogallol, and catechol. The pyrogallol andcatechol derivatives are. preferably substituted with an alkanoylradical having 6 18 (preferably 6-l6) carbon atoms, e.g. decanoyl,lauroyl, myristoyl, or hexadecanoyl. The preferred gallic acidderivatives are the esters of gallic acid and an alkanol of 6 18(preferably 6-16) carbon atoms. All of these polyhydroxyaromaticcompounds form strongly colored complexes with coreactant organic andinorganic iron salts such as ferric chloride, ferric nitrate, and ferricsalts of aliphatic or aromatic carboxylic acids such as ferric octoate,ferric benzoate, etc. Invisible entries printed according to thisinvention can therefore be developed by applying one of thesecoreactants to the printed surface with a suitable applicator. Thecoreactant can be dissolved in a solvent and applied from a fiber-tippedpen or rubbed on with frictional action using a wax crayon containingthe coreactant.

It is of course preferred that the color-forming, ironcomplexingreactant be light or pale in color, e.g. an off-white or gray color.Clueing can, in any event, be minimized by matching the colorof the inkformulation as closely as possible to the color of the paper. Thus,

pale yellow paper can be used with pale yellow ink, gray paper with grayink, etc., provided the ink itself is color stable in the environment ofthe printed page. A particularly suitable reactant from a colorstandpoint is lauryl gallate (dodecyl gallate), which is off-white incolor but gives a strongly colored gray-brown image when reacted with asuitable coreactant iron salt. As pointed out previously, thecolor-forming reactant should be ground or milled to form finely dividedparticles in the size range characteristic of lithographic ink pigments.Particles larger than 10 microns, and even larger than 5 microns are notcarried very well by a vehicle; consequently, the numerical average sizeof the color-forming reactant particles will be within the range ofabout 0.5 to 5 microns, preferably within the range of 1-3 microns.

The iron-inhibiting compound, hydrocarbon oil, wax, and flatting agentadditive previously referred to will now be described in detail.

The hydrocarbon oil. Aliphatic hydrocarbon petroleum distillates havinga KB. (Kauri-butanol) value below 28 and a boiling point above 200C. arepreferably included in the ink formulation to adjust the viscosity andtack of the ink and/or, in the case of oil-soluble quick-set vehicles,as a solvent or extender. Up to about 5% by weight (based on the totalink composition) can ordinarily be used. Hydrocarbon oils having aboiling range located entirely above 225 C. are preferred. An example ofa relatively low-boiling petroleum distillate is the so-called 440 oil,which boils in the range of 444 4 493 F. (229 256 C.). Higher boilingoils such as 535 oil are preferred; these oils generally have boilingranges above 275 C.

The wax. Any suitable wax used in lithographic inks can be added toadjust tack consistency and to decrease film-forming tendencies of theink. For example, up to 3% by weight of the ink formulation can be aparticulate polyethylene-type wax.

The iron inhibiting agent. When the color-forming reactant is milled incontact with iron or steel-containing rolls, or in contact with othermetals that can have undesirable chemical effects upon the color-formingreactant, it is preferred to add at least 0.5%, preferably at least0.7%, by weight of a compound having the ability to form a colorless orsubstantially colorless complex with iron. A particularly suitableexample of such a compound is anhydrous citric acid. The citric acidreacts with any foreign iron forming a colorless reaction product in theprocess of mixing, milling, and also after printing on the writingsurface. If desired, the citric acid may be combined with phosphoricacid for this purpose.

The flatting agent is particularly useful to reduce the glossiness ofthe printed ink when a high percentage of quick-set vehicle is used. Theinks of this invention typically comprise more than 55% by weightquick-set vehicle to ensure good pigment-carrying capacity. Atproportions higher than about 65%, the printed ink may have excessivegloss, even when some flatting agent is used. (A further disadvantage ofthese high proportions of quick-set vehicle is the relatively slow rateof invisible image development.)

The flatting agent. The preferred flatting agent is finely dividedmicron-size or sub-micron size silica particles. Preferably. the silicamakes up no more than about 5% by weight of the total ink composition.In addition to providing flatting effects which reduce the glossiness ofthe invisible ink deposits, the silica improves rub resistance andadjusts the physical properties of the ink to obtain ink stability onthe press. Synthetically produced silica is available in a wide varietyof particle sizes ranging from a fraction of a micron to severalmicrons, 3 micron-size silicas being preferred.

Other recommended additives. Whether or not the quick-set vehicle is apolyurethane alkyd, some additional liquid polyurethane alkyd, up toabout 4% of the total ink composition, is preferably added for viscosityadjustment, added stability on the press, and better rub resistance forthe ink once printed on the paper. Amounts of highly fluid polyurethanealkyds in excess of 4% or 5% result in too much fluidity over a periodof time so that the printed ink migrates into the paper fibers and is nolonger available for reaction with the coreactant metal salt applied tothe paper surface. Surprisingly, however, when the quick-set vehicleitself is a polyurethane alkyd and contains stiff material as well asthin material, the migration of printed invisible ink into the paperfibers is sufficiently small to avoid any difficulty in developing theinvisible image after print mg.

Another desirable additive for the inks of this invention is arelatively long chain organic compound having a polar group at one endof the chain. Suitable compounds of this type are the long chainaliphatic carboxylic acids such as oleic acid. These carboxylic acidsprovide faster inking of the lithographic printing plate.

The ink formulations of this invention contain no active drying agentsbecause of the inherent property of the color-forming reactant to tie upoxidizing agents, e.g. the cobalt and manganese salts commonly used asdriers in lithographic inks. The formulations of this invention dry byabsorption primarily, although some very slow evaporation of thealiphatic hydrocarbon oil can occur over a period of 3 to days, orlonger. Because of this absorption or filtration drying method, theratios of color-forming reactant, vehicle, and the other additives areimportant to the objective of obtaining a printable ink that will drysufficiently and not change with time because of penetration into thepaper fibers. The drying rate of the invisible ink formulation has,however, been found to be somewhat less critical than that of black andcolored inks.

The relatively small amount of invisible ink laydown, combined with theinherent circumstance that the latent print content of a printed sheetof this invention is usually a small percentage of the area of thesheet, has the effect of requiring close control of the amount of inkmetered to the ink train from the ink supply fountain of the offsetpress. It is difficult to maintain a water/ink balance with this lowlevel of ink being supplied and printed. Two methods of solving thisproblem are (1) to include a solid bar or block of invisible inkcontent, preferably near an edge of the paper, and covering at least onepercent of the surface area of the sheet, for example, a /2 X 8 inch barcan be printed at the bottom of a standard 8 /2 X 1 1 inch sheet; (2) toinclude a screened-in background, Le. a pattern of scattered dots ofinvisible ink content. The screened background area should surround anylatent images on the sheet having informational content, thereby alsohelping to reduce clueing by camouflaging the latent informationalcontent. A screened background area covering generally up to about 15%and in no event greater than 30% of the latent print informationalcontent camouflages this content without interfering with the vividcontrast of the activated or developed invisible image. Furthermore, thescreening allows the pressman to carry more ink on ink rollers on thepress, because more ink is being applied per sheet of paper. Anexcessive amount of invisible ink in the informational content of theinvisible entries can make it possible for one to read the latent imagecontent before development with a coreactive metal salt.

For quick set vehicles having a viscosity at room temperature greaterthan 500 poises, the optimum colorforming reactant content in the inksof this invention is 25 31% by weight. Higher amounts have poorstability in the ink train, and lower amounts provide slower imagedevelopment and, after development, relatively low color density.

The following non-limiting examples illustrate the principle andpractice of this invention. All parts are by weight unless otherwiseindicated.

Example 1 The following formulation illustrates the use of aphenolic-based quick-set vehicle containing a drying oil. The vehicle,Kemset 300-101 quick-set varnish, has a solids content of 62% by weight,a specific gravity of 0.939, a viscosity of 350 poises, and acid numberin the range of 5 10 mg KOI-I per gram. This quick-set vehicle isextendible with 535 oil, but can be used as the sole vehicle where speedof drying is more important. In this Example, the Kemset was modifiedwith SKP- 104 (Sun Chemical Corp), a polyurethane alkyd of solids and aspecific gravity of 1.00 1*: 0.05. Its acid number is 2 mg KOI-I pergram.

Parts by Weight Ingredient 61.0 Kemset 300--101 Varnish Sun ChemicalCorp.

2.0 SK? 104" (Polyurethane Alkyd) Sun Chemical Corp. 1.0 Micropoly"800-1 11 (Polyethylene type wax) Sun Chemical Corp. 31.5 Lauryl Gallate0.8 Syloid 72", Grade 73 micron-sized silica flatting agent (W. R. Grace& Co.) 1.7 Citric acid, anhydrous 1.2 Magic Brosv 535 oil 0.8 Oleic acid"Micropoly" 800-111 wax is a finely powdered modified polyethylene-typewax having a Gardner (Hellige) color of 1-2, and a particle size range01'545 microns.

Example 2 Parts by Weight Ingredient 63.0 Uroset" quick-set vehicleLawter Chemicals, lnc. 3.0 SK? 104 (Polyurethane Alkyd) Sun ChemicalCorp. 1.0 Micropoly 800-1 ll (Polyethylene type wax) Sun Chemical Corp.28.5 Lauryl gallate 0.5 Syloid 72, Grade 73 (4 micron sized) silicaflatting agent (W. R. Grace & Co.) l.7 Citric acid, anhydrous 1.5 MagicBros. 535 oil 0.8 Oleic acid 100.0

Examination sheets printed with this lithographic invisible ink and asecond, visible ink composition, were printed on offset paper of about0.02% iron content, based on the weight of the paper. The surface ofthis paper was toothy a rough vellum finish having a Sheffieldsmoothness number in the range of 240 270. These examination sheetsshowed no evidence of clueing over an extended period of time. Coated orcalendared glossy, smooth sheets had reflectance differences, however,such that the invisible image was visible without developing. Thus,rough and matte-like finishes on the paper were shown to be preferable.

What we claim is:

l. A quick set ink for providing printed, invisible entries on anabsorbent offset grade paper-like sheet, said quick set ink comprising:

1. at least 10% by weight of a color-forming component consistingessentially of a solid particulate polyhydroxyaromatic iron-complexingcolorforming reactant having the formula wherein n is selected from 0and l, and R represents a radical selected from the group consisting ofa monovalent aliphatic radical of greater than 4 carbon atoms,+CO-l-OR', and -t-COl-R', wherein R is a monovalent aliphatic radical ofgreater than 4 carbon atoms, the numerical average particle size of theparticles of said color-forming reactant being within the range of about0.5 to about 5 microns, and, as a vehicle for said particles,

2. at least 40% by weight of a color stable, two phase, quick setvehicle free of metallic driers having tack, viscosity, hydrophobicity,and pigment carrying capacity suitable for use in conventional wetlithographic ofiset printing, and

3. 0-3% by weight of wax; said quick set ink being hydrophobic andhaving a viscosity in the range of 3003,5 OOpoise at room temperature,the most volatile component in said quick set ink having a boiling pointgreater than 200C.

2. A quick set ink according to claim I wherein said color-formingreactant is a gallic acid ester of an aliphatic alcohol containing 6 18carbon atoms.

3. A quick set ink according to claim 1 wherein said composition furthercomprises a second ironcomplexing compound capable of forming a stable,substantially colorless iron-containing complex.

4. A quick set ink according to claim 1 wherein said compositioncontains an aliphatic hydrocarbon oil having a boiling point greaterthan 200 C. and a Kauributanol value less than 28.

5. A quick set ink according to claim 1 wherein said quick set vehiclecomprises a polyurethane alkyd having a viscosity greater than 500poise.

6. A printed form comprising:

1. an offset grade, absorbent paper-like sheet containing less than0.05% iron, based on the weight of said sheet,

2. a printed visible entry, and

3. a dry printed invisible entry derived from printing on said sheet aquick set ink comprising:

a. at least 10% by weight of a color-forming component consistingessentially of a solid particulate polyhydroxyaromatic iron-complexingcolorforrning reactant having the formula:

wherein n is selected from 0 and l, and R represents a radical selectedfrom the group consisting of a monovalent aliphatic radical of greaterthan 4 carbon atoms, i-CO+OR', and -l-CO-)-R, wherein R is a monovalentaliphatic radical of greater than 4 carbon atoms,

the numerical average particle size of the particles of saidcolor-forming reactant being within the range of about 0.5 to about 5microns, and, as a vehicle for said particles,

b. at least 40% by weight of a color stable, two phase, quick setvehicle free of metallic driers having tack, viscosity, hydrophobicity,and pigment carrying capacity suitable for use in conventional wetlithographic offset printing, and

0. 03% by weight of wax; said quick set ink being hydrophobic and havinga viscosity in the range of 3003,500 poise at room temperature, the mostvolatile component in said quick set ink having a boiling point greaterthan 200C.

7. A printed form according to claim 6 wherein said paper-like sheet hasa Sheffield smoothness number greater than and an iron content of lessthan 0.03% based on the weight of said sheet.

8. A printed form according to claim 6 wherein said paper-like sheetcomprises a fiber mixture containing a random distribution offluorescent fibers.

9. A printed form according to claim 6 comprising a plurality of saidprinted invisible entries, one of said printed invisible entries being asolid block covering at least about 1% of the area of said paper-likesheet.

10. A printed form according to claim 6 comprising a plurality of saidprinted invisible entries, at least one of said entries being an imagewith informational content surrounded by a discontinuous pattern of dotscovering up to about 15% of the area surrounding said image, said dotscomprising said color-forming reactant and said quick set vehicle.

11. An invisible quick set ink for providing printed, invisible entrieson an absorbent offset grade paper-like sheet, said quick set inkcomprising,

1. 20-35% by weight of a color-forming component consisting essentiallyof a particulate gallic acid ester of an alkanol of 6-18 carbon atoms,said particulate ester having a numerical average particle size in therange of 0.5- microns,

2. at least 55% by weight of a flowable color-stable two phase, quickset vehicle free of metallic driers having tack, viscosity,hydrophobicity, and pigment carrying capacity suitable foruse inconventional wet lithographic offset printing,

3. at least 0.5% by weight of an iron-complexing agent comprising citricacid,

4. an aliphatic hydrocarbon oil having a boiling point greater than225C. and a Kauri-butanol value less than 28,

5. O3% by weight of a particulate wax, and

6. 0-5% finely divided silica; said quick set ink being hydrophobic andhaving a viscosity in the range of about 300 to about 3,500 poise atroom temperature, the most volatile component in said ink having aboiling point greater than 200C. and components (l)(6) of said ink beingsufficiently well blended to prevent settling or separation of any ofsaid components.

12. An invisible quick set lithographic offset ink for providing printedinvisible entries on an absorbent offset grade paper-like sheet,said'quick set ink comprismg:

l. at least 40% by weight of a flowable color-stable two phase, quickset vehicle free of metallic driers having tack, viscosity,hydrophobicity, and pigment carrying capacity suitable for use inconventional wet lithographic offset printing, and, dispersed throughoutsaid quick set vehicle,

2. at least 10% by weight of a color-forming component consistingessentially of a light colored, solid, particulate, water-insolublecolor-forming reactant having a numerical average particle size withinthe range of about 0.5 to about 5 microns which reactant is capable offorming a strongly colored complex with a coreactant metal salt, saidcolorforming component being substantially free of any said correactantmetal salt, said color-forming reactant being sufficiently dispersedthroughtout said quick-set vehicle to prevent any substantial setting ofsaid color-forming reactant, and said quick set ink being hydrophobicand having a viscosity in the range of about 400 to about 3000 poise atroom temperature, the most volatile component in said quick set inkhaving a boiling point greater than 200C.

1. A QUICK SET INK FOR PROVIDING PRINTED, INVISIBLE ENTERIES ON ANADSORBENT OFFSET GRADE PAPER-LIKE SHEET, SAID QUICK SET INKCOMPRISING:
 1. AT LEAST 10% BY WEIGHT OF A COLOR-FORMING COMPONENTCONSISTING ESSENTIALLY OF A SOLID PARTICULATE POLYHYDROXYAROMATICIRON-COMPLEXING COLOR-FORMING REACTANT HAVING THE FORMULA
 2. AT LEAST40% BY WEIGHT OF A COLOR STABLE, TWO PHASE, QUICK SET VEHICLE FREE OFMETALLIC DRIERS HAVING TACK, VISCOSITY, HYDROPHOBICITY, AND PIGMENTCARRYING CAPACITY SUITABLE FOR USE IN CONVENTIONAL WET LIGHOGRAPHICOFFSET PRINTING, AND
 3. 0-3% BY WEIGHT OF WAX; SAID QUICK SET INK BEINGHYDROPHOBIC AND HAVING A VISCOSITY IN THE RANGE OF 300-3,400 POISE ATROOM TEMPERATURE, THE MOST VOLATILE COMPONENT IN SAID QUICK SET INKHAVING A BOILING POINT GREATER THAN 200*C.
 2. at least 40% by weight ofa color stable, two phase, quick set vehicle free of metallic driershaving tack, viscosity, hydrophobicity, and pigment carrying capacitysuitable for use in conventional wet lithographic offset printing, and2. A quick set ink according to claim 1 wherein said color-formingreactant is a gallic acid ester of an aliphatic alcohol containing 6 -18 carbon atoms.
 2. at least 55% by weight of a flowable color-stabletwo phase, quick set vehicle free of metallic driers having tack,viscosity, hydrophobicity, and pigment carrying capacity suitable foruse in conventional wet lithographic offset printing,
 2. a printedvisible entry, and
 2. at least 10% by weight of a color-formingcomponent consisting essentially of a light colored, solid, particulate,water-insoluble color-forming reactant having a numerical averageparticle size within the range of about 0.5 to about 5 microns whichreactant is capable of forming a strongly colored complex with acoreactant metal salt, said color-forming component being substantiallyfree of any said correactant metal salt, said color-forming reactantbeing sufficiently dispersed throughtout said quick-set vehicle toprevent any substantial setting of said color-forming reactant, and saidquick set ink being hydrophobic and having a viscosity in the range ofabout 400 to about 3000 poise at room temperature, the most volatilecomponent in said quick set ink having a boiling point greater than200*C.
 3. a dry printed invisible entry derived from printing on saidsheet a quick set ink comprising: a. at least 10% by weight of acolor-forming component consisting essentially of a solid particulatepolyhydroxyaromatic iron-complexing color-forming reactant having theformula:
 3. at least 0.5% by weight of an iron-complexing agentcomprising citric acid,
 3. A quick set ink according to claim 1 whereinsaid composition further comprises a second iron-complexing compoundcapable of forming a stable, substantially colorless iron-containingcomplex.
 3. 0-3% by weight of wax; said quick set ink being hydrophobicand having a viscosity in the range of 300-3,500 poise at roomtemperature, the most volatile component in said quick set ink having aboiling point greater than 200*C.
 4. A quick set ink according to claim1 wherein said composition contains an aliphatic hydrocarbon oil havinga boiling point greater than 200* C. and a Kauri-butanol value less than28.
 4. an aliphatic hydrocarbon oil having a boiling point greater than225*C. and a Kauri-butanol value less than 28,
 5. 0-3% by weight of aparticulate wax, and
 5. A quick set ink according to claim 1 whereinsaid quick set vehicle comprises a polyurethane alkyd having a viscositygreater than 500 poise.
 6. A printed form comprising:
 6. 0-5% finelydivided silica; said quick set ink being hydrophobic and having aviscosity in the range of about 300 to about 3,500 poise at roomtemperature, the most volatile component in said ink having a boilingpoint greater than 200*C. and components (1)-(6) of said ink beingsufficiently well blended to prevent settling or separation of any ofsaid components.
 7. A printed form according to claim 6 wherein saidpaper-like sheet has a Sheffield smoothness number greater than 160 andan iron content of less than 0.03% based on the weight of said sheet. 8.A printed form according to claim 6 wherein said paper-like sheetcomprises a fiber mixture containing a random distribution offluorescent fibers.
 9. A printed form according to claim 6 comprising aplurality of said printed invisible entries, one of said printedinvisible entries being a solid block covering at least about 1% of thearea of said paper-like sheet.
 10. A printed form according to claim 6comprising a plurality of said printed invisible entries, at least oneof said entries being an image with informational content surrounded bya discontinuous pattern of dots covering up to about 15% of the areasurrounding said image, said dots comprising said color-forming reactantand said quick set vehicle.
 11. An invisible quick set ink for providingprinted, invisible entries on aN absorbent offset grade paper-likesheet, said quick set ink comprising,
 12. An invisible quick setlithographic offset ink for providing printed invisible entries on anabsorbent offset grade paper-like sheet, said quick set ink comprising;